The invention relates to bandgap circuits, and more particularly, to bandgap reference circuits capable of generating bandgap voltage without varying temperature and manufacturing variations.
In integrated circuits, while reference generators are required output voltages thereof are typically fixed at 1.23V and are not applicable in low voltage operation.
FIG. 1 shows a conventional reference voltage generator with temperature compensation. As shown, the reference voltage generator includes a PMOS transistor M11, three resistors R10˜R13, an operational amplifier OP11, bipolar junction transistor (BJT) Q12, and eight parallel connected BJTs Q11. The voltage VBE1 is generated between the emitter terminals and the base terminals of the BJTs Q11, and a current IC1 (not shown) flows through each BJT Q11. The voltage VBE2 is generated between the emitter terminals and the base terminals of the BJTs Q12, and the current IC2 flows through the BJT Q12. The PMOS transistor M11 includes a source terminal coupled to an operating voltage VCC, a gate terminal coupled to an output terminal of the amplifier OP11, and a drain terminal coupled to the resistor R13. The resistor R10 has a first end coupled to the resistor R11 and the positive input terminal of the operational amplifier OP11, and the other end coupled to the emitter terminals of the parallel connected BJTs Q11. The resistor R12 includes one end coupled to the resistors R11 and R13 and the other end coupled to the negative input terminal of the amplifier and the emitter terminal of the BJT Q12.
The operational amplifier OP11 includes a positive input terminal coupled to the connection (node A) between the resistors R10 and R11, and a negative input terminal coupled to the connection (node B) between the resistor R12 and the emitter terminal of the BJT Q12. The operational amplifier OP11 normalizes the voltages on the nodes A and B, and generates a bandgap voltage VBG at the connection between the resistor R13 and the drain terminal of the PMOS transistor M11.
            V      BG        =                  V        BE2            +                                                  V              T                        ⁢                          ln              ⁡                              (                                                      I                    C2                                                        I                    C1                                                  )                                                          R            10                          ⁡                  [                                                                      R                  11                                                  R                  12                                            ×                              R                12                                      +                                          (                                  1                  +                                                            R                      11                                                              R                      12                                                                      )                            ⁢                              R                13                                              ]                      ,            wherein      ⁢                          ⁢              V        T              =          KT      q        ,  andthe parameter VT is a positive temperature coefficient. Thus, the voltage across the resistors R12 and R13 has a positive temperature coefficient, and the voltage VBE2 a negative temperature coefficient. Consequently, a stable voltage VBG unaffected by temperature and manufacturing variations is obtained.
The reference voltage VBG with temperature compensation, however, is limited to 1.23V because the negative temperature coefficient is a constant. Thus, this conventional reference circuit cannot provide required reference voltage for low voltage operation.